A variety of very-large-scale-integrated (VLSI) circuits of practical importance include transistor devices with short channel lengths and with shallow source and drain pn junctions. Typically, electrical interconnections for the transistors are supplied by first and second metallization levels which are insulated from each other by a second level insulating layer, such as a phosphorus-doped glass layer.
For example, metal-oxide-semiconductor (MOS) transistors integrated in a silicon semiconductor body can have channel lengths corresponding to gate electrodes whose lengths are as short as about 1.0 micron or less. In such transistors the source and drain junction depths, beneath the front (top) surface of the silicon body, are advantageously rather shallow, typically about 0.3 micron or less, in order to achieve desired transistor device performance characteristics, especially high speed of operation. Such shallow junctions, in the case of n-channel MOS transistors, are ordinarily achieved by doping the source and drain regions in the silicon body with the impurity arsenic rather than phosphorus, which diffuses more rapidly than arsenic. As the pn junction depths become that shallow, however, the problem arises that metals such as aluminum included in contact window portions of the device, to make electrical access connections from the second level metallization to the source and drain regions in the semiconductor, will undesirably penetrate down through the junctions during required heating steps and consequently will degrade the performance of the transistors. In practice, the aluminum has been known to penetrate through junctions having depths less than a micron. To counteract this problem, a polycrystalline silicon (polysilicon) contact electrode layer is deposited in the contact windows prior to the deposition of the aluminum metal thereon, whereby this polysilicon contact layer suppresses the penetration of the aluminum to the junction. The polysilicon contact electrode layer is typically doped with phosphorus to supply both a desirably higher electrical conductivity to the polysilicon and a desirable source of phosphorus for gettering of impurities during relatively high temperature gettering treatments typically at a temperature of 1000 degrees C. When using aluminum to metallize the source/drain electrodes for the required integrated circuit interconnections, this gettering at high temperature must be performed prior to laying down the aluminum, because of the relatively low melting temperature (about 660 degrees C.) of the aluminum. Although phosphorus diffuses in the silicon body about four times as fast as arsenic, nevertheless the phosphorus in the polysilicon contacts do not adversely affect the shallow source and drain regions in the MOS transistors, because the distance of closest approach of each such contact from the nearest such junction is equal to at least the minimum linewidth of the lithographic patterning procedures that are used to define the line and space features of integrated circuits.